As we know, multi-copter crashed from time to time due to motor failure. More often than not, people couldn’t figure out the root cause in the end. Based on our experience, it is possible something (e.g. small screw, nut, pin, etc.) getting stuck inside an out-runner motor for some reasons. This did happen that a screw nut was let into the motor when people were doing the assembly in the workbench.

Mostly out-runner brushless motors have an open shell and strong magnets inside, thus could capture something small but ferromagnetic (made of iron/nickel/cobalt) into motor unintentionally. And sometimes this is hardly to be found in advance.

The best way to prevent that a foreign object get into the motor unwittingly is to put a cover on the motor.